Filters or presses are important in industry, particularly in the manufacturing industry and the chemical industry in such processes as removing dirt impurities and other foreign matter from oils and liquids in grinding, cutting and forming operations or from cooling and treating waters. Filters or presses are also important in water and wastewater treatment plants to reduce the vast quantities of sludge currently produced.
Filters have been used for centuries to capture or separate solids from liquid streams, wherein either the separated liquid or solid is the desired product. Separation of the solid from the liquid or just the reduction of the liquid fraction from the slurry will result in the reduction of the large quantity of material to be transported to a landfill or to incinerate. The filter may be simply a porous screen which the slurry is subjected to or the apparatus may be a mechanical device which is complicated and expensive to purchase and operate.
Of all the mechanical dewatering or liquid-solid separation devices known today, the belt filter press is probably the most efficient and the bag filter is the simplest. The belt press, however, has not been without its limitations. One such problem has been the sealing of the belts to form a fixed volume. A problem in any type of moving belt filter has been the tendency of the liquid being filtered or dewatered to pass around the edges of the filter belt element. Conventional belt presses are open along their lateral edges and utilize the tendency of the slurry to thicken along these edges to form a self-sealing boundary therealong, as well as along the bottom of the column of the sludge in an upstanding belt press. However, these self-sealing boundaries are insufficient to maintain the slurry within the confines of the belts when subjected to moderately high hydrostatic head pressure or mechanical pressure. Consequently, the self-sealing features of the belt presses are a limitation on the amount of pressure which may be applied to the slurry entrained therein. Improved constructions which seals the edges have been developed but these seals are mechanical and subject to wear, failure, and are costly to provide and maintain. Known sealing devices include elastomeric members, magnetic sealing closure, conventional zippers and pressure closures. Patents in the U.S. covering such improvements are as follows: U.S. Pat. Nos. 3,630,381; 4,425,842; 3,965,011; 3,464,557; 4,475,453; 4,276,822; 4,040,952; and 4,514,301. Foreign patent documents which are noted improvements are as follows: Great Britain Pat. No. 2,031,293 and West Germany Pat. Nos. 2,751,849 and 3,404,422.
Furthermore conventional belt presses display a tendency to have their belts walk or travel in an unwanted direction as they move over their associated rollers, thereby making it difficult to maintain proper alignment of the belts without the use of pneumatic/hydraulic operated aligning mechanisms. Such alignment mechanisms are available, however, they are expensive to supply and operate and subject to failure. Prior art for belt alignment also includes roller chain alignment mechanisms attached to the media edges to help in belt alignment but this method is also complicated, costly and subject to failure.
One of the oldest filters used in industry and is still utilized today is the bag filter. In 1824, William Cleland, an Englishman, took out patents for the clarification of sugar juices through bags. Such bags were closed at one end by fastening with string while the other end was tied to an inlet nipple where the liquid to be filtered finds its way by gravity. Clarified filtrate was collected outside the bag as the bag retains the solids. When such bags are used today and a bag becomes clogged, it is removed, disconnected, emptied, cleaned and replaced, necessitating a very considerable amount of time and labor. Bag filters are therefore expensive to operate even though they give a clean filtrate. Consequently, one will find bag filters in only older industrial plants today.
Batch filter bags have been recently utilized in wastewater treatment plants in the thickening/dewatering of sewage sludges. These bags are utilized either singularly or arranged alternately one behind the other. However emptying of such devices is time-consuming and labor intensive. Yet another problem is that the hydraulic head acting on the sludge-slurry in the bag is not uniform, i.e., the maximum head is on the sludge in the bottom of the bag while the sludge in the upper portions of the filter bag will have far less hydrostatic head pressure therefore cake or filtered solid product emptied from such bags is inconsistent in thickened mass or cake dryness. It has been found that the use of such filter bags does not lead to a sufficient degree of removal of water or liquid, i.e., maximum dryness and minimum bulk, even if the full sludge bag hangs for several days prior to emptying due to the lack of maximum hydrostatic head acting on the entire depth of sludge. In some cases, the hydrostatic head pressure even in the bottom of the batch filter bag is insufficient for dewatering.
From the foregoing it is desirable to find an apparatus that would eliminate the limitations of the moving belt filter such as belt sealing and complex construction. Likewise it would be desirable to make the batch filter bag automatic for continuous sludge/slurry input and dewatered product withdrawal or ejection thus saving labor costs and initial cost associated with multiple units. Also it is desirable to have all of the sludge/slurry in the filter bag subjected to the same hydrostatic head as the sludge/slurry positioned at the bottom of the batch filter bag. That is to say, all of the sludge/slurry should be subjected to the maximum hydrostatic head available so that the dewatered or thickened product withdrawn or ejected is consistent in nature. Also it would be desirable to exert more pressure on the sludge in a bag filter over and beyond that of the hydrostatic head such as mechanical pressure or shear stresses upon the sludge in the bag filter if possible.
In the West German Pat. No. 2,751,849, means were provided to take advantage of the hydrostatic head generated by a vertical standing filter using similar principles to the batch filter bag but continuous with means to subject the slurry/sludge to the maximum hydrostatic head as the sludge moves down or through the device. The device disclosed therein is automatic or continuous with two parallel endless filter belts which run in the vertical plane facing each other between two perforated plates. The belts pass through compression rolls and are mechanically sealed at their sides or edges by rubber supports on chain belts to prevent lateral leakage as the belts advance. These chain belts also serve to move the filter medium and the compression rolls but, the disadvantage of the seals used in any type of moving belt filter, such as cited above, persist in the West German patent in that these seals are still mechanical and are subject to failure. The seals must be engaged to seal the sides of the belts at the start of the filter cycle as the belts advance and must be disengaged at the discharge end of the filter or completion of the filtration cycle to permit slurry introduction and cake discharge and filter media return to the top of the filter to begin a new cycle. As with any mechanical seal, these seals are subject to wear and failure. Furthermore, the mechanism of U.S. Pat. No. 2,751,849 is also very complicated and expensive to purchase and operate.
Thus the known prior art remains limited in its efficiency and economy of application.